We DON'T know how much testosterone the person in question is taking (along with any other drugs).

The issue is that there isn't some firm delineation between a physiological and supraphysiolgical dose/blood level. Likewise there isn't a firm cutoff as to what moves into the realm of performance enhancing. The 'normal' reference range for the T level of a male of any age is going to be huge. And dose response isn't an exact science. Two people can take the same dosage and metabolize the drug very differently yielding very different blood levels. It's very difficult to replicate a 'normal' output of testosterone production via supplementation. And again, it's almost impossible to define what a normal output is in a healthy male. I'm guessing this is at least part of the reason why sports governing bodies ban medical TRT across the board because there's just too much gray area here.

I would say it's pretty much impossible for anyone to prove or disprove that the person in question has derived any type of performance enhancement from their hormone therapy.